High in specific strength and specific rigidity, fiber reinforced composite materials, carbon fiber reinforced composite materials among others, are useful and have been used in a wide variety of applications including aircraft structure members, windmill blades, automobiles' exterior plates, and computer parts such as IC trays and notebook computer housing, and demands for them have been increasing every year.
A carbon fiber reinforced composite material has a nonuniform structure produced by molding a prepreg consisting essentially of carbon fiber, i.e., reinforcement fiber, and a matrix resin, and accordingly, such a structure has large differences in physical properties between the alignment direction of the reinforcement fiber and other directions. For instance, it is known that the interlaminar toughness, which represents the resistance to interlaminar fracture of the reinforcement fiber layers, cannot be improved drastically by simply increasing the strength of the reinforcement fiber. In particular, carbon fiber reinforced composite materials containing a thermosetting resin as matrix resin are generally liable to be fractured easily by a stress caused in a direction other than the alignment direction of the reinforcement fiber, reflecting the low toughness of the matrix resin. In this respect, various techniques have been proposed aiming to provide composite materials that have improved physical properties, including interlaminar toughness, to resist a stress in directions other than the alignment direction of the reinforcement fibers while maintaining high compressive strength in the fiber direction under high temperature and high humidity conditions, which is required for manufacturing aircraft structural members.
Furthermore, fiber reinforced composite materials have recently been applied to an increased range of aircraft structural members, and fiber reinforced composite materials are also in wider use for windmill blades and various turbines designed to achieving improved power generation efficiency and energy conversion efficiency. Studies have been made to provide thick members produced from prepreg sheets consisting of an increased number of layers as well as members having three-dimensionally curved surfaces. If such a thick member or curved-surfaced member suffers from a load, i.e., tensile or compression stress, the prepreg fiber layers may receive a peeling stress generated in an antiplane direction, which can cause opening-mode interlayer cracks. As these cracks expand, the overall strength and rigidity of the member can deteriorate, possibly leading to destruction of the entire member. Opening-mode, that is, mode I, interlaminar toughness is necessary to resist this stress.
In addition, the molding of a structural member of such a large size is liable to differences in heat history among different portions. Accordingly, it is also required for such a fiber reinforced composite material to maintain an undeteriorated shape and characteristics even if some fluctuations take place in the temperature-time profile during the molding process.
Compared to this, there is a proposal of a technique that uses high-toughness particle material of, for example, polyamide provided in regions between fiber layers so that the mode II interlaminar toughness will be increased to prevent damage to the surface that may be caused in falling weight impact test (see patent document 1). Even this technique, however, cannot serve adequately in the case of mode I interlaminar toughness.
Aside from this, another document has disclosed a material that contains a matrix resin composed of thermoplastic particles with a high melting point and thermoplastic particles with a low melting point and has high toughness against interlaminar fracture in addition to impact resistance (see patent document 2). However, it is difficult even for this technique to produce materials having both high mode I interlayer toughness and high compression strength in the fiber direction under moist heat conditions, and the resulting fiber reinforced composite materials suffer variations in interlayer morphology attributable to the melting and deformation of the interlayer particles depending on the molding conditions, failing to develop interlayer toughness stably. Furthermore, another document has disclosed a technique to use a combination of two types of particles with different glass transition temperatures (Tg) to produce a material having improved impact resistance and interlayer toughness while maintaining high heat resistance, and given some examples that use a combination of perfectly spherical polyamide particles with different Tg and particle diameters (see patent document 3). However, it is impossible even for this technique to develop interlayer toughness stably because the resulting fiber reinforced composite materials suffer variations in interlayer morphology attributable to the melting and deformation of the interlayer particles depending on the molding conditions.